The hematopoietic system comprises dynamic populations of blood cells, including erythrocytes, lymphocytes, monocytes, polymorphonuclear cells, e.g. basophils, eosinophils, and neutrophils; and megakaryocytes. All of these diverse cells are derived from the pluripotential hematopoietic stem cell. Through a series of cell divisions, there is a differentiation to one of these lineages. It is believed that, at least in vivo, after each cell division the developmental potential of the daughter cells is either maintained or further restricted relative to the parent, never expanded. One therefore observes that pluripotential stem cells give rise to multi-lineage committed progenitor cells, which give rise to specific lineages and finally mature cells. The coordinated changes of cellular properties leading to irreversible restriction of lineage commitment may be due to sequential activation or silencing of various genes.
Unfractionated mouse bone marrow has been shown to contain multiple types of colony-forming units (CFU), including multipotent CFU for all myeloid cells (CFU-GEMM or CFU-Mix), bipotent CFU for granulocytes and macrophages (CFU-GM), for megakaryocytes and erythrocytes (CFU-MegE), as well as monopotent CFU for granulocytes (CFU-G), macrophages (CFU-M), erythrocytes (CFU-E), or megakaryocytes (CFU-MK).
Three different populations of myeloid progenitors have been individually characterized: common myeloid progenitors (CMP), granulocyte/monocyte progenitors (GMP) and megakaryocyte/erythrocyte progenitors (MEP). CMPs give rise to all myeloid lineages whereas GMPs and MEPs give rise to cells in the granulocyte/monocyte and megakaryocyte/erythrocyte lineages respectively. The restricted differentiation capacity and lineage commitment of these cells have been clearly demonstrated by both in vitro culture and in vivo transplantation assay by Akashi et al. (2000) Nature 404, 193-197; and Na Nakorn et al. (2002) J. Clin. Invest. 109, 1579-1585. Whether monopotent progenitors for each myeloid lineage exist in the same manner is yet to be proven since they have not been prospectively isolated and tested for the in vitro and in vivo differentiation potentials at the clonal level.
In the megakaryocytic lineage, it is widely believed that CFU-MK are derived from monopotent megakaryocyte-committed progenitors (MKP) in the bone marrow. See, for example, Burstein et al. (1979) Blood 54, 169-179; Long et al. (1982) J. Cell. Physiol. 112, 339-344; Long et al. (1984) J. Clin. Invest. 74, 1686-1692; and Paulus et al. (1982) Proc. Natl. Acad. Sci. USA 79, 4410-4414.
The use of lineage committed progenitor cells circumvents many of the problems that would arise from the transfer of mature cells, and provides a means of screening for agents with highly specific activity. However, such progenitor cells must be separated from other hematopoietic cells. Separation requires identification of the cell and characterization of phenotypic differences that can be utilized in a separation procedure. Cells that are amenable to genetic manipulation are particularly desirable.
Relevant Literature
A number of review articles have been published addressing the phenotype of cells in hematopoietic lineages. Overall development of the hematolymphoid system is discussed in Orkin (1996) Curr. Opin. Genet. Dev. 6:597-602. The role of transcriptional factors in the regulation of hematopoietic differentiation is discussed in Georgopoulos et al. (1997) Annu. Rev. Immunol. 15:155-176; and Singh (1996) Curr. Opin. Immunol. 8:160-165. The phenotype of hematopoietic stem cells is discussed in Morrison & Weissman (1994) Immunity 1, 661-673; Spangrude et al. (1988) Science 241, 58-62; Enver et al. (1998) Blood 92, 348-351; discussion 352; Uchida et al. (1994) Blood 83, 3758-3779; Morrison et al., The aging of hematopoietic stem cells. Nat Med 2, 1011-1016 (1996).